1. Field of the Invention
The invention relates to metallic substrates having a deformable, especially cold-deformable, vitreous coating, to a process for their production and to their use.
2. Discussion of Background Information
Vitreous coatings on metal substrates such as steel or aluminum are generally produced by means of enameling processes. To this end, a particulate composition is applied to an adhesive layer applied beforehand and melts on heating to give a glass. In order to obtain continuous, impervious layers, however, the layer thicknesses generally have to be >50 μm. As a result of this, the layers, though, become inflexible and brittle, they are sensitive to bending, shock and impact, and flake off. Thermal injection processes too are incapable of producing deformable layers.
Various investigations on the application of thin inorganic layers to steel surfaces by means of the sol-gel process have already been carried out. For example, attempts have been made to apply zirconium dioxide layers to stainless steel in order to improve the corrosion resistance. Borosilicate glass layers have also been investigated. However, it was found that the refractory systems (high-melting oxides such as ZrO2) do not lead to impervious layers via these techniques and that the borosilicate glass layers can be applied only in layer thicknesses significantly below 1 μm, so that sufficient mechanical and chemical protection is not ensured. These processes have therefore not gained any industrial significance.
It is also known that transparent, crack-free and also one-dimensionally flexible vitreous layers with layer thicknesses in the lower μm range can be produced on metal surfaces with specific alkali metal ion-containing SiO2 coatings sols which comprise colloidal SiO2 particles; see DE 19714949 A1 and DE 10059487 A1. The nanoparticles present in the coating sols used there can be added to the sol from an external source or else generated in situ. The layers are applied with customary techniques and, after a drying step at temperatures up to 500° C., thermally densified. The thermal densification can be effected either in air or alternatively in oxygen-free nitrogen atmosphere. The coatings thus produced are only suitable for industrial applications in which a limited hydrolytic stability and no moisture-sensitive attrition resistance are required and in which the visual appearance plays only a minor role.
For example, it has been found that layers on stainless steel which have been densified at 500° C. under air have good scratch resistance (<150 μm in the IEC test with 20 N and tungsten carbide tip, radius 1 mm), but, on closer inspection, they have readily discernible discolorations in different gray to brown tones which may occur more or less homogeneously or else inhomogeneously, i.e. in a blotchy manner. Moreover, it is found after boiling for about 1 hour in tap water that the scratch resistance falls significantly (≧230 μm in the IEC test with 20 N and tungsten carbide tip, radius 1 mm), the layer changes its appearance and may peel off partially from the substrate.
On the other hand, it is found that layers densified at 500° C. under N2 atmosphere do not show the slight discolorations described above but have very poor chemical stability.
In both cases, cold deformation of the coating, for example on stainless steel sheet, is not possible. The layers burst even at bending radii of <5 mm (two-dimensional).
It is apparent that these unsatisfactory properties of the coatings are based on an inhomogeneous, unsuitable and irreproducible microstructure of the layers. An exact analysis is not possible, since the layers are vitreous and hence X-ray-amorphous. The structural units are within the lower nm range and, owing to their chemical composition (alkali metal silicate) are very low in contrast even for ultra-high-resolution transmission electron microscopy.
Owing to these unsatisfactory properties, serious disadvantages arise in the subsequent processing of coated metal substrates. For example, it is not possible to carry out any thermoforming-like reshaping processes. Equally, use in hot aqueous media is only possible to a limited extent.
It is therefore an object of the present invention to provide metallic substrates having a deformable vitreous coating with improved mechanical and chemical properties.